It is known to produce ((mono-trichloro)-tetra-(monopotassium dichloro))-penta-isocyanurate by reacting either tripotassium cyanurate with chlorine or potassium dichloroisocyanurate with hydrogen chloride or trichloroisocyanuric acid in aqueous medium at a temperature of 0.degree. to 50.degree. C. (Symes U.S. Pat. No. 3,150,132). It is also known that ((mono-trichloro)-tetra(monopotassium-dichloro))-penta-isocyanurate is formed if potassium dichloroisocyanurate monohydrate and trichloroisocyanuric acid are brought together as solids under high pressure (Berkowitz U.S. Pat. No. 4,118,569). In each case, the starting material is cyanuric acid or at last cyanuric chloride. The materials provided for the reaction must first be produced in partly expensive manner from the cyanuric acid or the cyanuric chloride. The yields of ((mono-trichloro)-tetra-(monopotassium-dichloro-))-penta-isocyanurate based on the cyanuric acid or the cyanuric chloride as starting material are below 80% and therefore are unsatisfactory.